1. Field of the Invention
The present disclosure relates to a zoom lens and an image pickup apparatus, such as a video camera, a monitoring camera, a digital still camera, and a broadcast camera, equipped with the same.
2. Description of the Related Art
In recent years, an image pickup apparatus, such as a video camera, a digital still camera, a broadcast camera, and a monitoring camera using a solid-state image sensor, and a camera using a silver-halide film, tends to be miniaturized in its entire size with a high functionality. In a photographic optical system used in such an image pickup apparatus, it is demanded that a zoom lens have a short total length, a high zoom ratio, and a high resolution. As a zoom lens satisfying such demands, there is known a positive-lead type zoom lens in which a lens unit having positive refractive power is located on the most object side.
As a positive-lead type zoom lens, there is known a zoom lens including a first lens unit having positive refractive power, a second lens unit having negative refractive power, and a third lens unit having positive refractive power in order from an object side to an image side.
In Japanese Patent Application Laid-Open No. 2006-171655, there is discussed a zoom lens including four lens units having positive, negative, positive, and positive refractive power in order from an object side to an image side. In addition, in Japanese Patent Application Laid-Open No. 2009-047903, there is discussed a zoom lens including five lens units having positive, negative, positive, positive, and positive refractive power in order from an object side to an image side. Furthermore, in Japanese Patent Application Laid-Open No. 2008-281927, there is discussed a zoom lens including five lens units having positive, negative, positive, negative, and positive refractive power in order from an object side to an image side.
In general, in order to obtain a zoom lens having a high zoom ratio while achieving miniaturization in the entire zoom lens, it is desirable to increase the refractive power (optical power, that is, an inverse of the focal length) of each unit of the zoom lens and reduce the number of lens elements. However, in such a zoom lens, an aberration fluctuation caused by zooming increases. Therefore, it is difficult to obtain excellent optical performance over the entire zoom range.
In the positive-lead type zoom lens described above, in order to obtain excellent optical performance and miniaturize the entire zoom lens with a high zoom ratio, it is important to appropriately set the refractive power of each lens unit or a refractive index of a material used in the lens. Particularly, it is important to appropriately set the refractive power of the second lens unit or the refractive index of a negative lens included in the second lens unit.